The electroless deposition of cobalt-phosphorus films emphasizing such magnetic properties as coercivity, hysteresis loop squareness and remanence is well known in the art. The coercive force of a magnetic material, typically measured in oersteds, is significant because it is a measure of the opposing magnetic intensity that must be applied to a magnetic material to remove the residual magnetism when the material has been magnetized to saturation. The magnetized material, for example, can be used in rigid magnetic storage discs, hard bias layer for MR thin film heads, magnetic detector tag applications (bias or shunt layers).
In making a magnetic layer suitable for use in a rigid magnetic storage disc, electroless Co(P) deposited from solution containing cobalt sulfate, sodium citrate, boric acid, sodium hypophosphite and sodium hydroxide results in a hard magnetic material possessing a coercivity (H.sub.c) of 510 Oe. In accordance with the present invention, if varying amounts of a sulfur containing compound, such as sulfamic acid, is added to the electroless plating solution having the composition described above, the coercivity can be varied and soft magnetic materials having H.sub.c as low as 1.6 Oe can be obtained.
U.S. Pat. No. 3,138,479 discloses Co(P) coercivity in the range of 400-450 Oe which was obtained by varying the pH and agitation. The plating bath includes cobalt chloride, ammonium chloride, sodium citrate and sodium hypophosphite.
U.S. Pat. No. 3,378,400 discloses nickel and cobalt coatings and their alloys deposited upon metallic and non-metallic surfaces and discloses the use of sulfamic acid and nickel or cobalt salts thereof in the metallic plating solution. No magnetic properties are disclosed in this reference.
U.S. Pat. No. 3,360,397 discloses coercivity of Co(P) in the range of 25-850 Oe; the coercivity was varied by changing the concentration of two complexing agents in the deposition solution.
U.S. Pat. No. 3,423,214 discloses electroless deposition of ferromagnetic metals such as cobalt and cobalt based alloys and specifically reports on coercivity of Co(P) in the range 110-235 Oe. The coercivity in this reference was varied by changing the concentration of the complexing agent in the deposition solution.
U.S. Pat. No. 3,447,657 discloses coercivity of Co(P) in the range of 350-950 Oe which was also varied by changing the concentration of the complexing agent.
U.S. Pat. No. 3,523,823 discloses a "soft" magnetic memory film of the nickel-cobalt type electrolessly plated onto a ceramic substrate. The reference discusses the magnetic properties of alloys and discloses coercivities in the range of 0.5-15 Oe.
U.S. Pat. No. 4,150,172 discloses variation of pH of the plating solution results in varying coercivity of Co(P) from 300-1000 Oe.
U.S. Pat. No. 4,659,605 discloses varying temperature, time of deposition and pH of the plating solution results in Co(P) coercivity from 300-1000 Oe.
IBM Technical Disclosure Bulletin 01-62, p.52-53 discloses coercivity in the range from 40 to 700 was obtained by varying concentration of complexing agent or by adding Ni.sup.+3 as the alloying ion.
IBM Technical Disclosure Bulletin 03-64, p. 65-66 discloses coercivity in the range from 150 to 900 was obtained by addition of Fe.sup.+2 as the alloying ion.
IBM Technical Disclosure Bulletin 07-65, p.209 discloses coercivity in the range from 50 to 600 Oe was obtained by varying pH using ammonia or a buffering agent such as sodium tetraborate.
The present invention differs from the prior art disclosures cited above because the coercivities of Co(P) of the present invention span a wide range starting from softer magnetic material with 1.6 Oe to a hard material of 550 Oe. The Co(P) of the present invention includes both soft and bard magnetic materials. The prior art references recognized only Co(P) in the range of hard or soft but not the possibility for making both types of materials from a single bath.
Variation in the coercivity of Co(P) in this invention was achieved by changing the concentration of a non-reactant in the deposition solution such as sulfamic acid, H.sub.3 NO.sub.3 S or sodium sulfate. In the references cited above, variation in coercivity was achieved by changing the concentration of reactants (complexing agents, --OH, [pH]), temperature or agitation or both. In the present invention, by varying the concentration of a non-reactant, it was possible to control the structure of the solution/substrate interphase (the electrical double layer) which provides a control of the Co(P) crystallization process and coercivity of the Co(P) deposit.